1. Field of the Invention
This invention relates to apparatus for comminuting polymer gels to obtain a defined, uniform particle size using a cutting unit comprising a static cutting element with or without a dynamic cutting element and shape-stable feeding of the polymer gel to the cutting unit, and a process for performing the comminution.
2. Description of Related Art
Water-containing polymer gels are obtained from water-soluble monomers in the course of the production of water-soluble or water-swellable polymers and are used in a very wide variety of fields. They are used for example as flocculation aids, drainage and retention aids, as viscosity-increasing agents in aqueous media, for example in tertiary oil production, as grinding or dispersing assistants, adhesives, wastewater treatment agents, as superabsorbents in the hygiene and sanitary sector or as soil improvers in agriculture, as sealing agents in the building construction industry and also in the production of electricity- and light-conducting cables or in medicine, for example to lower the cholesterol level by binding bile acids or bile acid salts or in the treatment of dialysis and predialysis patients to bind phosphates.
The production of the polymers or polymer gels, for example acrylic and allylic polymers, such as acrylic acid, methacrylic acid, hydroxyethylmethacrylic acid and acrylamide homopolymers and their derivatives or copolymers composed of a major fraction of acrylic acid, methacrylic acid, hydroxyethylmethacrylic acid and acrylamide derivatives and other copolymerizable or crosslinking monomers, of polymer gels based on native or chemically modified proteins (eg gelatin and derivatives thereof) or of polymer gels based on natural or chemically modified homo- and heteropolysaccharides such as for example starch and cellulose, agarose, carageenan, chitosan, xanthan, guar gum, alginate, pectinate, sucrose gels, and also of polymer gels based on polyelectrolyte-sucrose gels, and also of polymer gels based on polyelectrolyte complexes, such as copper alginate for example, of polymer gels based on hydrolyzed crosslinked maleic anhydride copolymers (eg crosslinked hydrolyzed, partly neutralized maleic anhydride-methyl vinyl ether copolymers or maleic anhydride-styrene copolymers), of N- or amino- or ammonium-containing polymers having cationic groups and suitable counterions, which can contain hydrophobic groups, where appropriate, is generally effected by bulk polymerization, suspension polymerization, emulsion polymerization or solution polymerization, as described for example in EP-A1-068 189, EP-A1-0 415 141, EP-A-0 374 709, WO 00/38664, WO 99/33452, WO 99/22721, WO 98/43653, U.S. Pat. No. 5,496,545, EP-A1-0 366 986, etc.
The polymer gels in question can be produced in 2 different ways.
a) By polymerization and partial crosslinking in a single step.
b) By subsequent crosslinking of a synthetic or natural polymer or of derivatives thereof.
The prior art polymerization reaction is followed by appropriate crosslinking (gelling). This provides, as a function of the monomers and crosslinkers used and/or of the polymerization parameters, polymer gels which are water-containing, soft and rubbery or which are brittle and extremely shear sensitive.
The processing of these polymer gels to form powders is effected in the subsequent process or workup steps, such as for example coarse comminution, fine comminution, polymer-analogous chemical reaction, washing, separation, drying and grinding etc. and hitherto represented an appreciable cost and inconvenience if the polymer properties achieved in the gel state, such as for example swelling properties and particle structure, and associated processing properties, such as for example chemical convertibility, sedimentation capability, filtration speed, drying speed and grindability, were to be preserved. Thus, as described in WO 96/36464 for example, the requisite uniformity of the processing sequence was already impaired by the precomminution and comminution of the soft, rubbery or brittle gels, since rubbery gel blocks or gel strands are for example torn apart by kneaders into nonuniformly sized pieces, whereas the division of soft gels produces with increasing plasticity gel portions having ever larger dimensions and the kneading tools are often blocked by gel portions which have become wrapped thereon. This gives rise to an uneven flow of material, which leads to different layer thicknesses for example in foraminous belt drying and hence to an insufficient or excessive drying of the polymer with impairment of the subsequent grinding and classifying operation and which, on the other hand, for example by hornification or by the partial thermal degradation of the polymer, gives rise to a reduction in quality, for example in swellability, the generation of toxic gases etc, and hence to inferior performance on the part of the product. In lieu of kneaders it is generally also customary to use extruders, for example a meat grinder, to comminute polymer gels, by forcing the gel by means of a conically narrowing single-screw conveying system through a breaker plate. However, this system is absolutely unsuitable for pressure- and shear-sensitive polymer gels, since the gel—even in the case of built-in rotating cutting blades—is more sheared than cut apart, which gives rise to enormous problems in the washing and the subsequent separation of the gels.
The literature, for example DE 35 39 385, DE 35 06 534 or WO 96/36464, discloses further comminuting processes and apparatuses, which, however, either have a very costly and inconvenient construction and/or imply a complicated, costly or inconvenient comminuting operation or which are suitable for coarse division only and in no instance lead to a uniform particle distribution.